Piracy is hampering the delivery of food aid to Somalia, forcing relief agencies to use aircraft or less convenient ports that lengthen delivery time, the African Development Bank's chief economist said on Thursday.

Mthuli Ncube said concerns over piracy have not lessened as the international community steps up its relief effort for around 12 million people in the Horn of Africa in need of emergency aid as a result of drought, exacerbated by conflict in Somalia.

"Absolutely, piracy is very much a concern and not abating at all," Ncube told the Guardian. "It hampers the delivery of food aid. Some has to be flown in, which has an impact on cost, or it has to go to ports like Mombasa, Kenya, and then be driven overland, which takes time."

A new report by the African Development Bank (AfDB) said piracy has been a longstanding problem for aid efforts to Somalia as 80%-90% of food aid arrives by sea. In 2007, the World Food Programme (WFP), the UN's food aid agency, reported that the number of ships willing to carry food aid had been cut by half because of the increased dangers faced by ships in Somali waters.

However, increased security measures have had an impact. The number of reported pirate attacks in east Africa dropped from 222 in 209 to 172 last year, according to the International Maritime Organisation, which monitors piracy. Most of the attacks occurred off the coast of Somali and in the Gulf of Aden.

For the time being, humanitarian aid continues to arrive in Mogadishu by sea and air, including aid sent by many Gulf and Arab states, amid warnings that famine in Somalia has not peaked and that hundreds of thousands of people face imminent starvation and death without a massive global response. The UN says it has received $1.1bn, just 46% of the $2.4bn requested from donor countries.

Catherine Bragg, the UN deputy emergency relief co-ordinator, warned the UN security council on Wednesday that the situation is likely to worsen, given very high levels of acute malnutrition and under-five mortality, a continued increase in cereal prices, and a below-average rainy season harvest.

High food prices are compounding problems in the Horn. The Food and Agriculture Organisation (FAO) reported on Wednesday that cereal prices in east Africa reached new peaks in several countries last month.

High prices of cereals such as sorghum and maize in the region are the result of a combination of factors, including drought, reduced secondary season harvests this year and high fuel prices that have driven up transport costs, said the FAO. In Somalia, where famine has been declared in five areas in the south-central region, prices of domestically produced staples, sorghum and maize, showed some signs of decline last month but remain between 150% and 200% higher than July last year.

Luca Alinovi, the FAO's representative in Kenya, told the security council the organisation was working to prevent Somalis from abandoning their drought-stricken farms by paying them cash for small jobs, as those who leave can become dependent on aid.

Ncube said emergency aid and measures such as cash payments were vital now, but then long-term measures need to be taken to ensure sustainability. "What is required once the rains come is to take measures to support pastoralists, such as providing aid for them to buy cattle, something the bank is doing in east Africa," he said. "What is also needed is water management, like building dams."

Meanwhile, the UN's special representative for Somalia, Augustine Mahiga, has called for greater funding and logistical help, including aviation and mine disposal equipment, for Amisom, the African Union peacekeeping force. The AU has proposed increasing Amisom to 20,000 from its current strength of 6,200 to ensure government control of Mogadishu and expand it south to the borders of Kenya and Ethiopia and the town of Kismayo, areas still controlled by al-Shabaab and other militant groups.

Al-Shabaab last week announced its withdrawal from the capital, but Mahiga said the security situation remained precarious with the militants likely to resort to terrorist attacks and guerrilla tactics.